The poisoning effect of silicones on metal catalysts is known in the art. Polydimethylsilicone vapors are known to poison hydrotreating catalysts as described by Kellburg et al., "Deactivation of HDT Catalysts by Formation of Silica Gels from Silicone Oil characterization of Spent Catalysts from HDT of Coker Naphtha Using Si and C CP/MAS NMR," J. Catalysis 143, 45-51 (1993). The poisoning of volatile inorganic compound (VOC) catalysts, such as Pt/Alumina, by silicones in the ink/varnish industry has led to efforts to choose non-volatile silicones.
Silica modified catalysts for use in processes for the selective disproportionation of toluene to p-xylene are described in, for example, U.S. Pat. Nos. 4,477,583; 4,465,886; 4,145,315; 4,100,219; 4,851,604; 5,173,461; 5,243,117; and WO 93/17788.
In the manufacture of silica modified catalysts, volatile silicon-containing species in the effluent stream may plate out on downstream VOC catalysts and poison them. Therefore, it is an object of the present invention to avoid poisoning of downstream catalysts. It is a further object of the present invention to avoid damaging downstream equipment.